Implantable detection/stimulation multipolor microlead

ABSTRACT

Leads for use with implantable medical devices may be implanted in the venous, arterial, or lymphatic networks. The diameter of a microlead may be at most equal to 1.5 French (0.5 mm), and it may include a plurality of micro-cables each including: an electrically conductive core cable for connection to one pole of a multipolar generator of an active implantable medical device, and a polymer insulation layer surrounding the core cable. At least one exposed area may be formed in the insulation layer to form a detection/stimulation electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/052,371, filed Oct. 11, 2013, which claims the benefit ofand priority to French Patent Application No. 1259758, filed Oct. 12,2012, both of which are incorporated herein by reference in theirentirety.

BACKGROUND

The disclosure generally relates to the “active implantable medicaldevices” as defined by Directive 90/385/EEC of 20 Jun. 1990 of theCouncil of the European Communities.

This definition includes in particular cardiac implants for monitoringof the cardiac activity and for the generation of stimulation,defibrillation and/or resynchronization pulses, in case of arrhythmiadetected by the device, and/or for sensing electrical activity. It alsoincludes neurological devices, cochlear implants, medical substancediffusion pumps, implantable biological sensors, etc.

These devices may comprise a housing generally designated as a“generator”, electrically and mechanically connected to one or moreintracorporeal “leads” having electrodes for coming into contact withthe tissue (e.g., myocardium, nerve, muscle, etc.) in which it isdesirable to apply stimulation pulses and/or to collect an electricalsignal.

According to some exemplary embodiments, the present disclosure morespecifically relates to a detection/stimulation microlead forimplantation in the venous, arterial or lymphatic systems.

The current principle of electrical stimulation of tissues is based on adevice, usually called a “lead”, which is an object notably implantedthrough various venous, arterial or lymphatic vessels, and the functionof which is to transmit an electrical signal to the target tissue. Thelead may be used to effectuate one or more of the following properties:

-   -   Ease of implantation by the physician in a vessel network of the        patient, and especially ease of advancing the lead into the        vessels by pushing, making the lead follow tortuous branches and        crossings, and transmitting torque;    -   X-ray vision to allow the physician easy navigation through the        vessels of the network under X-ray fluoroscopy;    -   Atraumaticity of the lead in the veins, which requires a very        flexible structure and the absence of rigid transition or sharp        edges;    -   Ability to transmit an electrical signal to the tissues and to        stably perform monopolar or multipolar electrical measurements;    -   Biocompatibility with living tissue for a long-term        implantation;    -   Ability to connect to an implantable device, source of signal        transmission;    -   Ability to sterilize (gamma rays, temperature, . . . ) without        damage;    -   Biostability, especially corrosion resistance in the living        environment and resistance to mechanical stress fatigue related        to patient and organs movement, and    -   Compatibility with MRI imaging which is particularly important        in neurology.

The current architecture of the leads meeting these requirements can besummarized in a generally hollow structure to allow passage of a styletor a guidewire, and comprising components such as insulated conductorcables, connected to mechanical electrodes for providing electricalconductivity, radiopacity, etc.

These leads thus require a complex assembly of a large number of parts,wires and associated insulation, creating significant risk of breakagedue to mechanical stresses to which they are exposed in the long term.

Examples of such leads are given in U.S. Pat. Nos. 6,192,280 A and7,047,082 A.

Among the difficulties met, the management of stiffness gradientsrelated to mechanical parts used, which strongly affect theimplantability properties and mechanical strength over the long term(fatigue), can be cited. Furthermore, in terms of fatigue of assemblies,any stiffness transition zone may induce risks of fatigue, difficulty tosterilize due to the presence of zones difficult to access, and problemsof solidity of the conductor junctions at the connection with theelectrodes and the connector.

Moreover, the clinical trend in the field of implantable leads is toreduce the size to make them less invasive and easier to handle throughthe vessels. The current size of implantable leads is typically of theorder of 4 to 6 Fr (1.33 to 2 mm).

However, it is clear that reducing the size of the leads increasescomplexity and imposes technical constraints generating risks.

SUMMARY

One embodiment of the disclosure relates to a sensing/pacing multipolarlead configured for implantation into a venous, arterial, or lymphaticnetwork. The lead includes a plurality of microcables. Each microcableincludes an electrically conducting core cable configured to beconnected to a pole of a multipolar generator of an active implantablemedical device. The core cable includes a plurality of elementarystrands twisted together. The microcable further includes a polymerinsulation layer surrounding the core cable. The polymer insulationlayer has formed therein at least one exposed area. The core cableincludes at least one bare core cable portion exposed through the atleast one exposed area. The at least one bare core cable portion forms amonopolar detection/stimulation electrode. The plurality of microcablesare coupled together in a twisted configuration within the lead andtogether form a multipolar detection/stimulation distal active portionof the lead.

Another embodiment relates to a lead for an implantable medical device.The lead includes a plurality of microcables. Each microcable includes acore cable including a plurality of cable strands and an insulationlayer surrounding the core cable. The insulation layer has formedtherein at least one exposed area, and the core cable includes at leastone bare core cable portion exposed through the at least one exposedarea. Each at least one bare core cable portion comprises an electrodeusable by the implantable medical device to perform at least one ofsensing and stimulation.

Yet another embodiment relates to an implantable medical device thatincludes a generator and a lead. The generator includes an electroniccircuit configured to perform at least one of a sensing operation and astimulation operation. The lead is configured to be connected to thegenerator and used by the generator in performing the at least one ofthe sensing operation and the stimulation operation. The lead includes aplurality of microcables. Each microcable includes a core cableincluding a plurality of strands and an insulation layer surrounding thecore cable. The insulation layer has formed therein at least one exposedarea, and the core cable includes at least one bare core cable portionexposed through the at least one exposed area. Each at least one barecore cable portion comprises an electrode of the lead.

Another embodiment relates to a method of forming a lead for use with animplantable medical device. The method includes providing a plurality ofmicrocables. Each microcable includes a core cable and an insulationlayer surrounding the core cable, and the core cable comprises aplurality of cable strands. The method further includes forming, foreach of the plurality of microcables, at least one exposed area withinthe insulation layer and exposing at least one bare core cable portionthrough the at least one exposed area. Each at least one bare core cableportion forms an electrode of the lead.

Another exemplary embodiment relates to a sensing/pacing multipolar leadfor implantation into a venous, arterial, and/or lymphatic network. Thelead includes a plurality of microcables. Each microcable includes anelectrically conducting core cable, for connection to one pole of amultipolar generator of an active implantable medical device, and apolymer insulation layer surrounding the core cable. The polymerinsulation layer includes at least one exposed area formed in theinsulation layer and forming a monopolar detection/stimulationelectrode. Each core cable is formed by a plurality of elementarystrands joined together in a respective strand of strands. The strand ofstrands of the different microcables are themselves assembled togetherin a strand of microcables. The strand of microcables forms amulti-polar detection/stimulation distal active portion of the lead. Thelead is a microlead, the diameter of said distal active portion being atmost equal to 1.5 French (0.5 mm).

In one or more of the above-identified embodiments:

-   -   the lead may have an outer diameter less than or equal to 1.5        French;    -   the distal ends of the microcables may be longitudinally        shifted (l) from each other, so as to produce a progressive        reduction of the diameter and to introduce a stiffness gradient        in the most distal portion of the microlead;    -   the lead may include a central microcable having a distal        electrode;    -   the core cable may be formed by a plurality of individual        strands of 0.033 mm diameter;    -   the core cable may have a diameter of 0.1 mm and may be formed        by a strand of seven elementary strands of 0.033 mm diameter;    -   the elementary strands may have a composite structure consisting        of a structuring material and of a radiopaque material;    -   in some embodiments, the structuring material is a stainless        steel, a MP35N cobalt alloy, a precious metal, titanium, or a        NiTi alloy;    -   in some embodiments, the radiopaque material is tantalum (Ta),        tungsten (W), iridium (Ir), platinum (Pt), or gold (Au);    -   the elementary strands may include an outer layer of material of        low magnetic susceptibility (e.g., lower than 2000×10⁻¹²        m³×mol⁻¹);    -   in some embodiments, the material of low magnetic susceptibility        is tantalum (Ta), titanium (Ti), rhodium (Rh), molybdenum (Mo),        tungsten (W), palladium (Pd), or gold (Au);    -   the insulation layer may have a thickness of 25 μm;    -   the polymer constituting the insulation layer may be a        fluoropolymer;    -   the lead may include a sleeve of heat-shrinkable polymer        partially surrounding the microlead;    -   the detection/stimulation electrode may include a conductive        ring fixed on a microcable exposed area by reduction of its        outer diameter;    -   the detection/stimulation electrode may include an inner        conductive ring fixed on an exposed area by reduction of its        outer diameter and an outer conductive ring fixed to the inner        ring;    -   the outer ring may be fixed to the inner ring by laser welding        or bonding using a conductive adhesive;    -   the detection/stimulation electrode may include a conductive        ring attached to an exposed area by adhesive bonding using a        conductive adhesive;    -   the conductive rings may include platinum-iridium;    -   the exposed area may extend 360 degrees over a length        corresponding to one turn of microcable strand; and/or    -   the lead may include a dipole including two 360 degree exposed        areas disposed face-to-face on two non-consecutive microcables.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, characteristics and advantages of the presentdisclosure will become apparent to a person of ordinary skill in the artfrom the following detailed description of the present disclosure, madewith reference to the drawings annexed, in which like referencecharacters refer to like elements and in which:

FIG. 1 is a partial perspective view of a detection/stimulationmultipolar microlead according to an exemplary embodiment of thedisclosure.

FIG. 2 shows the microlead of FIG. 1 before introduction of the outersheath and the electrodes.

FIG. 3 is a cross section of the view of FIG. 2 .

FIG. 4 is a sectional view of an elementary strand of a microcable shownin FIG. 3 .

FIG. 5 is a partial perspective view of a microlead realized accordingto a first embodiment of the disclosure.

FIGS. 6 a to 6 c are views illustrating the steps of introducing aninner ring on a microcable according to a second embodiment.

FIGS. 7 a-7 c are views illustrating the steps of attaching by laserwelding an outer ring on the inner ring of FIGS. 6 a to 6 c.

FIGS. 8 a-8 c are views illustrating the steps of gluing an outer ringon an exposed area of a microcable.

FIG. 9 is a partial perspective view showing an electrode in the distalportion of the central microcable of the microlead of FIG. 1 .

FIG. 10 is a partial perspective view of a microlead realized accordingto a third embodiment.

FIGS. 11 a and 11 b are views showing the realization of a dipole on amicrolead in accordance with an exemplary embodiment of the disclosure.

FIGS. 12 a and 12 b illustrate an improvement of the embodimentsdescribed in the preceding figures, achieving a gradual reduction indiameter in the distal end of the lead.

DETAILED DESCRIPTION

A reduction of lead diameter, to less than 1.5 French (0.5 mm), forexample, may open up prospects for medical applications in variousfields ranging from cardiology to neurology in the presence of a vein,artery or even lymphatic network such as the cerebral venous network orthe coronary sinus venous network.

Today, the technology of electrical stimulation has led to majoradvances in the field of neuromodulation, which is to stimulate targetareas of the brain for treatment of Parkinson's disease, epilepsy andother neurological diseases.

One could imagine what could be achieved with this type of technology toaddress new areas difficult to reach today, by small stimulation leadsor “microleads”, with great strength to ensure long-term biostability.With small microleads, it is possible to notably consider the passage ofanastomosis to implant a stimulation device of the left ventricle viatwo distinct areas.

Such a technique would also allow a less invasive approach to thesetherapies and especially superior efficacy of treatments. It would alsobe possible to connect one or more microleads through the consideredvessel network to the target location. Their implantation could be done,because of their small size, by guiding devices used in interventionalneuroradiology today for the release of springs (coils) in the treatmentof intracranial aneurysms. In particular, microleads of 1.5 French arecompatible with catheters with an internal diameter of 1.6 French.

Furthermore, there is a need to propose multipolar leads related to thefunction of “electronic repositioning” in applications. This offers manypossibilities for stimulation of tissue through the choices and thepolarization of certain lines of conduction from among the number oflines included in the microlead. This technique allows programming ofstimulation areas according to the therapy, a very interesting property,especially in neurology.

EP 2455131 A1 and its US counterpart US 2012/0130464 (Sorin CRM SAS)disclose a lead grouping a plurality of distinct monopolar microleads.The lead body that includes the microleads has a number of lateralopenings along its length from which successively emerge respectivemicrocables. Each of these microcables forms a monopolar line extendingin a counterpart vessel of the coronary network, thereby stimulatingthrough the same lead several vessels (each vessel being stimulated by amonopolar microcable), and thus covering a large area of the leftventricle. But considered separately, each of the microleads has onlyone electrical conductor, and, therefore, cannot be subjected to anelectronic repositioning.

A purpose of this disclosure, according to at least some exemplaryembodiments, is to provide a small microlead which would be consistentwith the general properties of implantable leads as they were listedabove. A microlead of the present disclosure may reduce complexity and,therefore, the final cost, and/or may allow a multipolar connection on asame microcable in order to independently polarize lines and stimulatetissue by means of electrodes disposed at different positions along themicrolead.

The size of the microlead may make it possible to access very smallveins inaccessible today with larger devices. The microlead may alsogreatly facilitate the navigability in venous, arterial and lymphaticnetworks due to its flexibility and small size.

In some embodiments, the microlead may be configured to meet a number ofrequirements.

In terms of reliability of the conductor/electrode mechanical andelectrical connection, the requirements may include avoiding designsthat could affect the immediate mechanical strength of the conductor byassemblies of electrical continuity components inserted betweendifferent portions of conductors. In some embodiments, it may be betterto utilize a physically continuous conductor. Maximum sealing may helpavoid exposing the conductors to body fluids.

In terms of performance, the requirements may include:

-   -   To give the microlead an outside diameter compatible with the        size of targeted vessels;    -   To not alter the local flexibility of the microlead to maintain        maximum maneuverability, including passage through the small        veins;    -   To ensure isodiameter profile to allow passage within an        implantable catheter, and veins; and    -   To facilitate and simplify the microlead/catheter assembly        process.

According to various embodiments of the disclosure, multipolardetection/stimulation microleads intended for implantation in venous,arterial and lymphatic networks are provided.

As disclosed in EP 2455131 A1 above, the lead may include a plurality ofmicrocables, each microcable including an electrically conductive corecable, for connection to a pole of a multipolar generator of an activeimplantable medical device active. The lead may also include aninsulation polymer layer surrounding the core cable. At least oneexposed area may be formed in the insulation layer and may form asensing/pacing electrode. Each core cable may be formed by a pluralityof elementary strands joined together in a respective strand of strands.

According to some embodiments, the lead of the disclosure is remarkablein that the different strands of microcables are themselves assembledtogether in a strand of microcables and form an active distal portion ofthe lead for multipolar stimulation. In some embodiments, the diameterof the distal active portion may be at most equal to 1.5 French (0.5mm).

The microlead may have high flexibility, enabling its manipulation bythe physician, especially during its implantation, when, for example, itis introduced it into vessel networks with high tortuosity and manybranches. The microlead may avoid injuries that could occur with muchmore rigid leads that are incompatible with tissue.

According to one embodiment, the core cable is formed by a plurality ofindividual strands of a diameter of 0.033 mm. In particular, a corecable of 0.1 mm is formed by a strand of seven elementary strands of0.033 mm diameter.

The choice of a stranded multiwire structure composed of very thinelementary strands to constitute the core cables increases theirresistance to mechanical fatigue due to patient and organ movement, theflexion breaking limit of a wire being substantially inverselyproportional to its diameter.

To enhance this important biostability property, it is advantageousthat, in some embodiments, the individual strands have a compositestructure consisting of a structuring material and of a radiopaquematerial. The structuring material may have a high intrinsic resistanceto fatigue, such as a stainless steel, a cobalt alloy (e.g., MP35N), aprecious metal, such as titanium or a NiTi alloy notably known under thename of nitinol, etc. To these metals responsible for ensuring themechanical qualities of the core cable is added a radiopaque material tomake the microlead visible to X-ray during its implantation by thephysician. In some embodiments, the radiopaque material can be selectedfrom the group consisting of: tantalum (Ta), tungsten (W), iridium (Ir),platinum (Pt) and gold (Au).

To make electrical contact with the tissues and transmit thedetection/stimulation electrical signals, the core cables may be used toform the electrodes of the microlead by exposing areas in an insulationlayer surrounding the cables.

In some embodiments, the insulation layer is a fluorinated polymer, forexample ethylene tetrafluoroethylene (ETFE), and has a thickness of 25μm.

According to some embodiments, the microlead includes a strand ofmicrocables (e.g., seven microcables). In some embodiments, aheat-shrinkable polymer sheath, for example of polyester (PET), maypartially surround the microlead.

To limit the heating of the core cable by skin effect during MRIimaging, the strands may include an outer layer of material of lowmagnetic susceptibility, below 2000×10⁻¹² m³×mol⁻¹. The material havinga low magnetic susceptibility may be, optionally, selected from thegroup comprising: tantalum (Ta), titanium (Ti), rhodium (Rh), molybdenum(Mo), tungsten (W), palladium (Pd) and gold (Au).

According to some embodiments, a detection/stimulation electrodeincludes a conductive ring attached to an exposed microcable area byreduction of its outer diameter.

To prevent stress on the ring, the sheaths and the strands, in someembodiments, a detection/stimulation electrode includes, first, an innerconductive ring secured to an exposed area by reduction of its outsidediameter, and, second, an outer conductive ring fixed on the inner ring.The rings may include, for example, platinum-iridium. The outer ring maybe fixed to the inner ring by laser welding or gluing using a conductiveadhesive.

The invention also discloses a detection/stimulation electrodecomprising a conductive ring secured to an exposed area by gluing usinga conductive adhesive.

In some embodiments, the exposed area extends 360° over a lengthcorresponding to a coil of a microcable strand. It may be possible tocarry out on the microlead at least a dipole consisting of two exposedareas on 360° arranged opposite on two nonconsecutive microcables.

Finally, in some embodiments, the distal ends of the microcables arelongitudinally offset relative to each other, so as to produce aprogressive reduction of the diameter and introduce a stiffness gradientin the most distal portion of the microlead.

The microleads of the disclosure may be detection/stimulation multipolarmicroleads intended to be implanted in venous, arterial and lymphaticnetworks. The diameter of the microleads, in some embodiments, may notexceed 1.5 French (0.5 mm). They may be particularly suited toapplications involving the function of electronic repositioning,mentioned above, which involves a plurality of separate and independentconduction lines, each connected to one pole of the generator of animplantable device. In some embodiments, the lines may each be connectedto one pole generator via an IS-1 or IS-4 connector for a cardiac lead,or even a larger number of poles for a neurological lead.

The microlead 50 shown in FIG. 1 includes seven microcables 40 ₁, 40 ₂,40 ₃, 40 ₄, 40 ₅, 40 ₆, 40 ₇ assembled in a strand as shown in FIG. 2 .Each microcable forms for the microlead 50 a conduction line connectedto a pole of the generator.

As shown in FIG. 3 , a microcable comprises a core cable 11 surroundedby an insulation layer 20 that provides electrical insulation betweenthe microcables.

In the embodiment shown in cross section in FIG. 3 , each core cable 11is formed by a strand of seven elementary wires 10 whose diameter is,for example, 0.033 mm. The diameter of a core cable 11 is then 0.1 mm.

In some embodiments, elementary strands 10 of the type shown in FIG. 4may be utilized. Such strands may include a core 1 of structuralmaterial such as stainless steel, an alloy of cobalt of the MP35Nseries, a noble metal, titanium, or a NiTi alloy, of high fatigueresistance. Such a core of structural material (e.g., having a diameterof 0.033 mm) may help achieve maximum tensile strength in the extremefatigue stress conditions to which such structures can be submitted.

To ensure sufficient visibility to X-rays for the implantation of themicrolead, it may be useful to introduce along the core cable a minimalamount of radiopaque material 2. Such a composite structure may exhibitboth fatigue resistance and radiopacity. Materials that may be used forradiopaque material 2 may include materials known for visibility toX-rays, for example, tantalum (Ta), tungsten (W), iridium (Ir), platinum(Pt) and gold (Au). Such materials may generally not have high fatiguestrength.

The compatibility of implantable devices with modern medical imagingtechniques such as MRI, is essential to ensure optimal patient care.

Because of its overall metal structure, the microlead is at risk ofoverheating due to “skin effect” induced currents outside the elementarystrands under the action of applied magnetic fields. However, due to thesmall diameter of the strands, heat dissipation is favored and theheating effects of MRI are reduced. Furthermore, the thermal energystored by the material, already limited in volume, can be furtherreduced if the individual strands are coated with an outer layer ofmaterial of low magnetic susceptibility (magnetic susceptibility beingthe ability of a material to be magnetized by the action of an externalmagnetic field).

In some embodiments, favorable materials may include those whosemagnetic susceptibility is less than 2000×10⁻¹² m³×mol⁻¹, such astantalum (Ta), titanium (Ti), rhodium (Rh), molybdenum (Mo), tungsten(W), palladium (Pd), and gold (Au).

In some embodiments, the thickness of the insulation layer 20 is 0.025mm (25 μm). Microcables of a diameter equal to 0.150 mm and a strand ofseven microcables of a diameter equal to 0.45 mm may be thus produced.

In various embodiments, characteristics required for the insulationlayer 20 may include:

-   -   Fatigue resistance,    -   Electrical insulation,    -   Long-term biocompatibility,    -   Biostability    -   Possibility of transformation and implementation compatible with        the conductor of the core cable.

To realize the insulation layer 20, materials with high chemicalinertness, such as fluoropolymers, which also have very good insulationproperties, may be preferred. Among these compounds, ETFE (ethylenetetrafluoroethylene) may in particular be mentioned.

Methods for producing the insulation layer 20 of the core cable are, forexample, co-extrusion of the conductor or heating of a heat shrinkabletube.

As can be seen in FIGS. 1 and 2 , in some exemplary embodiments, theinsulation layers 20 surrounding the core cables 11 of the microcableshave at least one exposed zone 30 to form a sensing/pacing electrode forthe microlead 50, such as the electrodes 52 of FIG. 1 . Such an exposedzone 30 may be formed according to manufacturing methods which will bedescribed in greater detail below. The exposed zones 30 are obtained bytechniques such as laser ablation.

In the case of stimulation in an anastomosis, for example, it is easilypossible to add one or more series of electrodes on each microcable, inorder to increase the number of stimulation points:

-   -   With one electrode per peripheral microcable, six peripheral        electrodes of the central microcable and one electrode at its        distal end are obtained, or seven poles and seven electrodes;    -   With two electrodes perperipheral microcable, twelve peripheral        electrodes of the central microcable and one electrode at its        distal end are obtained, or seven poles and thirteen electrodes;    -   With three electrodes per peripheral microcable, eighteen        peripheral electrodes of the central microcable and one        electrode at its distal end are obtained, or seven poles and        nineteen electrodes; and so on.

This example takes into account only one distal electrode at the end ofthe central microcable 40 ₇. It is, however, possible to place aplurality of electrodes if the microcable is elongated in its distalportion. In this case, there is no limit to the number of electrodes.For a microlead consisting of seven stranded microcables, the number ofpoles depends on the number of microcables, in the present example,seven poles.

Another possibility is to electrically connect a plurality ofmicrocables to the same electrical potential to increase reliability bythis redundancy.

It is thus possible, for example, with a 7×7 structure as illustrated inFIG. 3 , to connect two (or three) of the seven microcables to the samepole of the proximal side of the generator. On the distal side, theelectrodes of these microcables may be regrouped in the same stimulationzone to form a specific stimulation point. Conversely, the electrodesmay be moved several centimeters from each other to create a largestimulation surface.

Finally, FIG. 1 more particularly shows that the microlead 50 issurrounded by an outer sheath 51, except in areas occupied by theelectrodes 52. This sheath 51 may be of heat shrinkable polymer, such aspolyethylene terephthalate (PET). In some embodiments, its thickness maybe 0.025 mm (25 μm), which corresponds to a final diameter of 0.5 mm or1.5 French for the microlead 50.

FIG. 5 illustrates a first embodiment of the microlead 50 in which theconductive rings 52 of platinum/iridium 90/10 are respectively placednext to a exposed area (not shown in the figure). Each ring 52 is fixedby reducing its outer diameter, for example by crimping, ensuring thelocal contact of the inner diameter of the ring 52 with the core cable11 of the exposed microcable.

Reducing the diameter of the ring 52, obtained by deformation ofmaterial, results in stresses inherent to the ring but also to theinsulation layer 20 and to the core cables 11. The material of the rings52 may be sufficiently malleable to avoid damaging the nonexposed areas.

One method for assembly of the microlead 50 of FIG. 5 is toalternatively thread a shrink tubing length 51 and then a conductivering 52.

It should be noted that the electrodes 52 are annular electrodes andallow 360° stimulation.

FIGS. 6 a to 8 c disclose variants configured to ensure the integrity ofthe different insulation layers during the implantation of the electrode52.

In the embodiment of FIGS. 6 a to 7 c , a first ring 53 may be threadedonto one of the microcables 40 (FIG. 6 a ) to be positioned facing anexposed area 30 (FIG. 6 b ). The inner ring may also includeplatinum-iridium, and/or may have an inner diameter at least equal tothe outer diameter of the microcable (e.g., 0.15 mm) and/or an externaldiameter of 0.18 mm. The first ring 53 may be crimped (FIG. 6 c ) so asto make electrical contact with the core cable 11 and keep theisodiameter. The outer diameter of the inner ring 53 may be 0.15 mm.

This ring 53 may be used as the visible marker (FIG. 7 a ) underbinocular, in vertical position for the positioning (FIG. 7 b ) of asecond ring 52 (e.g., an outer ring or stimulation ring fixed to theinner ring 53).

This solution avoids subjecting the seven microcables to a crimpingforce. The electrical contact is then transferred between the two rings52, 53. The risks of damage by crimping on the insulation layers 20 ofthe microcables are thus reduced.

A hole 52 ₁ is formed in the outer ring 52 (FIG. 7 c ) to facilitate theorientation and positioning of rings 52, 53 for an assembly by laserwelding.

According to the embodiment of FIGS. 8 a-8 c , a particular microcable40 has a zone 30 without insulation layer 20 (FIG. 8 a ). This exposedarea is covered by a deposit of conductive adhesive 31 (FIG. 8 b ) formounting and positioning an outer ring 52, which is the stimulationelectrode, in contact with the tissues (FIG. 8 c ).

The function of the conductive adhesive is to transmit an electricalcurrent between the two conductor components and to preserve theintegrity of the insulation layers of the adjacent microcables. There isno risk of melting of the polymer of the insulation layers, or to cutthem or locally reduce their thickness.

A wide range of biocompatible adhesives commonly used in implantablemedical devices are available from Epoxy Technology, Inc.

This gluing technique can also be applied to the assembly of an innerring 53 and an outer ring 52, as described with reference to FIGS. 7 a-7 c.

The structure beneath the glued rings consists of rigid andnon-compressible metal strands and of insulating layers of soft, andtherefore compressible, polymer. On a microscopic scale, it is possibleto define this structure as flexible and possibly varying in size andgeometry. The use of glue is to compensate these microscopicdeformations due to the intrinsic characteristics of the glue and tointroduce a relative flexibility in the assembly of the two rings andmake this attachment less brittle when subjected to tensile, bending ortorsion stresses.

FIG. 9 shows a distal electrode 52 arranged at the end of the centralmicrocable 40 ₇ and manufactured according to one of the precedingembodiments, including the one involving an inner ring and an outerring.

FIG. 10 shows a version of the multipolar microlead wherein theelectrodes are directly formed by the elements of core cables 11. Forexample, the electrode 52 ₂ in FIG. 10 is the portion of the core cable11 exposed through the exposed area 30 of the microcable 40 ₂. In thiscase, each individual strand being a conductor, its material isbiocompatible, e.g. core 1 of MP35 and envelope 2 of platinum.

The strength of the structure is ensured by an alternation of elementsof sheath 51 surrounding the strand of seven microcables. Preferably,the length in the alternation of the outer sheath is greater than thelengths of the intervals. This sheath also plays an important role inthe handling of the microlead, during insertion into the catheter. Italso provides the shaping of the distal portion to reduce the risk ofmovement of the microlead in the veins.

It should be noted that in this configuration, each point of stimulationis angularly oriented and does not allow annular stimulation.

The electrode surface area is about 0.314 mm² for a length of 1 mm. Thissmall stimulation area is favorable to the longevity of the battery.

FIGS. 11 a and 11 b show a low power dipole consisting of spiralmicrocables and each covered, in this case, with a heat-shrink PETsheath. For the two microcables 40 ₁ and 40 ₄, first an exposed area 30of a length corresponding to one turn (360°) is made, thereby increasingthe conductive surface of each core cable 11 to a value equivalent tothat of an annular ring electrode.

The conductive surface thus twisted provides better contact with thetissues of the vein on 360°.

This architecture allows the manufacturing of a microlead that can beused as a dipole and thus get a very small distance between the poles.In this example, the distance between the poles is 0.15 mm, separated bya central microcable 40 ₇ of constant thickness.

The position of the two exposed microcables in the periphery may notablydiffer by alternation of nonconsecutive microcables, such as wrappedmicrocable/exposed microcable.

The advantage of such a device is to create an electric field betweentwo electrodes, at a very short distance, of the same surface, of thesame material, and located in the same electrolyte, which has the effectof increasing the intensity of the electric field.

FIGS. 12 a and 12 b illustrate an improvement of the embodimentsdescribed above, by shifting each end of the strand (and thus of themicrocable) e.g. l=1 to 5 mm from the end of the previous strand. Theresult is a gradual decrease in the diameter of the end of themicrolead, which, in this example, gradually changes from D=0.45 tod=0.15 mm, on a length of L=20 to 30 mm.

This longitudinal offset of the ends of the individual strands has twoadvantages:

-   -   First, it introduces in the most distal part of the microlead a        stiffness gradient avoiding an abrupt change in diameter between        the beam combining all microcables 40 ₁-40 ₇ and the central        microcable 40 ₇, which greatly reduces the risk of premature        failure of components due to bending mechanical stresses;    -   Second, it improves the electrical insulation between the        strands, and therefore the insulation of the different poles of        the microlead, while avoiding any immediate vicinity of non        insulated ends of the conductor cores of the different        microcables.

What is claimed is:
 1. A method of forming a lead for use with animplantable medical device, the method comprising: providing a pluralityof microcables, each microcable comprising a core cable and aninsulation layer surrounding the core cable, wherein the core cablecomprises a plurality of cable strands; and forming, for each of theplurality of microcables, at least one exposed area within theinsulation layer and exposing at least one bare core cable portionthrough the at least one exposed area, wherein the at least one exposedarea within the insulation layer is formed between a firstheat-shrinkable polymer sheath of the insulation layer and a secondheat-shrinkable polymer sheath of the insulation layer; wherein each atleast one bare core cable portion forms an electrode of the lead,wherein the lead has an outer diameter of no greater than 1.5 French. 2.The method of claim 1, further comprising longitudinally shifting distalends of the plurality of microcables from each other to produce aprogressive reduction of the diameter and to introduce a stiffnessgradient in a distal portion of the lead.
 3. The method of claim 1,further comprising positioning a central microcable in a center of theplurality of microcables, the center microcable having a distalelectrode.
 4. The method of claim 1, further comprising forming, foreach of the plurality of microcables, the core cable by twisting aplurality of elementary strands together.
 5. The method of claim 4,further comprising forming the elementary strands from a compositestructure comprising a structuring material and a radiopaque material.6. The method of claim 4, further comprising forming an outer layer ofthe elementary strands from a material of low magnetic susceptibility,wherein the magnetic susceptibility of the outer layer of material islower than 2000×10⁻¹²m³×mol⁻¹.
 7. The method of claim 1, wherein theinsulation layer comprises a fluoropolymer.
 8. The method of claim 1,further comprising coupling together the plurality of microcables in atwisted configuration within the lead such that the twistedconfiguration includes a center microcable and a plurality of peripheralmicrocables surrounding the center microcable and forming a multipolardetection/stimulation distal active portion of the lead.
 9. The methodof claim 8, further comprising extending the at least one exposed area360 degrees around a circumference of the lead over a lengthcorresponding to one turn of the microcable within the twistedconfiguration.
 10. The method of claim 9, further comprising forming adipole with two non-consecutive microcables of the plurality ofmicrocables, such that an exposed area of each of the two microcablesextends 360 degrees around the circumference of the lead and the exposedareas of the two non-consecutive microcables are disposed across fromone another within the twisted configuration.
 11. The method of claim 1,further comprising positioning an inner ring on each of the at least onebare core cable portion of each of the plurality of microcables.
 12. Themethod of claim 11, further comprising positioning an outer ring tosurround the plurality of microcables, wherein the outer ring is inelectrical contact with the inner ring.
 13. The method of claim 1,wherein each of the plurality of microcables is configured to beindividually conductive and individually-selectable by an implantablemedical device, for selective stimulation of the separately exposedareas.
 14. A lead for an active implantable medical device formed by aprocess comprising the steps of: providing a plurality of microcables,each microcable comprising a core cable and an insulation layersurrounding the core cable, wherein the core cable comprises a pluralityof cable strands; and forming, for each of the plurality of microcables,at least one exposed area within the insulation layer and exposing atleast one bare core cable portion through the at least one exposed area,wherein the at least one exposed area within the insulation layer isformed between a first heat-shrinkable polymer sheath of the insulationlayer and a second heat-shrinkable polymer sheath of the insulationlayer; wherein each at least one bare core cable portion forms anelectrode of the lead, wherein the lead has an outer diameter of nogreater than 1.5 French.
 15. The lead of claim 14, wherein the processof forming the lead further comprises longitudinally shifting distalends of the plurality of microcables from each other to produce aprogressive reduction of the diameter and to introduce a stiffnessgradient in a distal portion of the lead.
 16. The lead of claim 14,wherein the process of forming the lead further comprises couplingtogether the plurality of microcables in a twisted configuration withinthe lead such that the twisted configuration includes a centermicrocable and a plurality of peripheral microcables surrounding thecenter microcable and forming a multipolar detection/stimulation distalactive portion of the lead.
 17. The lead of claim 14, wherein each ofthe plurality of microcables is configured to be individually conductiveand individually-selectable by an implantable medical device, forselective stimulation of the separately exposed areas.